Set print symbol off in mi-var-display.exp

Set print symbol off in mi-var-display.exp

Hi,
I see the following fail on arm-none-eabi target,

-var-evaluate-expression -f nat foo^M
^done,value="0x3 <_ftext+2>"^M
(gdb) ^M
FAIL: gdb.mi/mi-var-display.exp: eval variable -f nat foo

the "<_ftext+2>" isn't expected in the test, so "set print symbol off"
can prevent printing it.  It is obvious and I'll commit it in three
days if no comments.

gdb/testsuite:

2014-09-09  Yao Qi  <yao@codesourcery.com>

	* gdb.mi/mi-var-display.exp: Set print symbol off.